Argonaut Octopus Mystery Solved

After centuries of speculation, biologists have documented one way a strange group of octopus-like creatures use their seashell-shaped cases.

Female argonauts, a group of four species that are close cousins of octopuses, grow delicate white shell-like cases. Biologists have found argonauts with air bubbles in their cases, and now it turns out the animals use the trapped air to float at a comfortable depth, says Julian Finn of Museum Victoria in Melbourne, Australia.

In the first reports from scuba observations of wild argonauts, Finn maneuvered Argonauta argo females so air escaped from their cases. The animals flailed as if struggling to maintain their orientation and quickly jetted to the water surface.

Once at the surface, argonauts rocked their cases and took on air, he says. Then they positioned body parts to seal in some of the air and jetted downward, leaving behind a trail of bubbles.

When the argonauts stopped several meters below the surface, water pressure compressed the remaining air inside the case enough that it counteracted the animals’ weight, leaving the argonauts floating neutrally buoyant at a chosen depth.

“Argonauts are fantastic animals to dive with,” Finn says, though he does acknowledge that “when they really got going, I couldn’t keep up with them.”

People have mused about the function of argonauts’ striking shell-like structures at least since Aristotle suggested that the animals sail or row them like boats.

Argonauts in the wild aren’t easy to find, and previous studies of captive argonauts, which raised the possibility that bubbles were bad for the animals, may have been muddled by the effects of keeping the creatures in aquaria, say Finn and Mark Norman, also of the Museum Victoria. Those tanks were probably too shallow to allow biologists to see the animals’ full behavior, the researchers say in their new analysis, posted online in Proceedings of the Royal Society B the week of May 17.

The idea that argonauts use their shells for buoyancy sounds plausible, says cephalopod biologist Michael Vecchione, a National Oceanic and Atmospheric Administration scientist based at the Smithsonian Institution in Washington, D.C. “I wonder how it would function during a storm at sea — and maybe it doesn’t,” he says.

Bubble trapping, however, may not be the only function of the shell-like case, he says. Female argonauts tuck masses of tiny eggs into spare space in the structure, Vecchione notes, much as bottom-dwelling octopuses protect their eggs in rock crevices.

Only female argonauts grow the shell-like structures, but males have very different bodies, presumably with different buoyancy issues. Males grow to about the size of the eye of a full-grown female and mate by sacrificing a detachable arm specialized for one-time delivery of sperm. Biologists at first mistakenly classified the remnant male arms as some kind of parasite that occasionally infected females’ encased body.